Atomic Number, Mass Number, Atomic Mass, Molecular Mass and Mole

Atomic Number is the number of Protons in nucleus of an Atom. Mass number of an element is the number of nucleons viz.  Neutrons and Protons. The atomic mass number is represented by a super index in left side such as ²³⁸U. The number 238 denotes that it has total 238 of nucleons (neutrons and Protons).

Difference between Atomic Mass and Mass Number

While mass number is total number of nucleons, atomic mass is relative mass of an atom to another. Earlier, atomic mass of Hydrogen was fixed at 1 without any units and other atoms were assigned masses relative to it. Currently, atomic mass is defined as 1/12 of a mass of an atom of the carbon-12 (¹²C) isotope. This is called atomic mass unit (amu) or a Dalton.  On this basis, the mass of Hydrogen atom is 1.0080 amu. Mass of oxygen – 16 (₁₆O) atom is 15.995 amu.

Nuclear Binding Energy

All the protons have a similar charge that is positive charge. To keep all the protons together which are similarly charged (positive), a nuclear force exists which is a very short range force and works within the nucleus. This is called Nuclear Binding energy and is released when nucleus is broken apart via nuclear fission. Thus, the energy required to break apart or split the nucleus of the atom into its component nucleons viz. Neutrons and Protons is also called Nuclear Binding Energy.

Due to nuclear binding energy the mass of the atomic nucleus is always less than the sum of the individual masses of the nucleons. This gives an explanation to the non whole number of the atomic mass. For example the mass of 19F is 18.9984032 u. This mass difference is called Mass defect. Please note that nuclear binding energy is millions of times more than the electron binding energy.

Molecular Mass

Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. For example, molecular mass of methane which contains one carbon atom and four hydrogen atoms can be obtained as follows:

Molecular mass of methane,

(CH4) = (12.011 u) + 4 (1.008 u)

= 16.043 u

Similarly, molecular mass of water (H2O)

= 2 atomic mass of hydrogen + 1 atomic mass of oxygen

= 2 (1.008 u) + 16.00 u

= 18.02 u

Mole

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the ¹²C isotope. It may be emphasised that the mole of a substance always contain the same number of entities, no matter what the substance may be. In order to determine this number precisely, the mass of a carbon-12 atom was determined by a mass spectrometer and found to be equal to 1.992648 10^–23 g.